1. Field of the Invention
This invention relates to an active type automatic focusing device (hereinafter called AF device) which uses a signal projecting means such as an infrared ray emitting diode (hereinafter called IRED) and more particularly to an AF device using an electromagnetic means such as an electromagnet of the pulling-holding type for actuating the AF device to start movement of a lens barrel or the like.
2. Description of the Prior Art
Heretofore, control over the actuation of an AF device such as the start of a lens barrel movement and the start of beam scanning of an IRED has been accomplished through the movement of a mechanical member which is interlocked with a shutter release button with a driving force obtained from a spring force or the like. In accordance with this conventional method, however, the AF device of the lens barrel and the IRED is unlocked even when the normal operations of a shutter device, a film winding device, etc. are no longer possible due to a drop in the voltage of a power source battery. Therefore, if the shutter release button is depressed under such a condition, the AF device operates resulting in abnormal operations of the shutter and the film winding device which operate subsequent to the AF device. To solve this problem, a method of accomplishing control over the actuation of the AF device by means of a magnet has been proposed. In this method, the AF device is actuated by a pulling force developed by a power supply to an electromagnet and movement of the lens barrel or the like is stopped by cutting off the power supply employing this method. In a voltage drop of the power source battery, the power supply to the magnet is inhibited by an electric circuit means so that the AF device is prevented from operating. Therefore, the problem mentioned above can be solved by this method.
Meanwhile, this prior art method has a shortcoming in that: a sufficient large pulling force is required in attracting an armature for actuating the AF device with the pulling force of the electromagnet. A large pulling or attracting current of several hundred mA must flow at the time of the armature attraction. After the attraction, the armature is held at a yoke. This holding action merely requires a holding current of several ten mA. Meanwhile, however, in an AF device using an IRED, it is necessary to start a distance measuring action by lightening up the IRED upon actuation of the AF device by the pulling or attracting force of the electromagnet. In accordance with the current level of technique, a lighting current of several hundred mA is required for lighting an IRED. Therefore, when the pulling current for the electromagnet and the lighting current for the IRED overlap each other, the battery sustains a great burden. This causes a quick drop in the battery voltage and tends to render other control circuits inoperative. This inconvenience conspicuously arises especially at low temperatures.